The input offset voltage of an operational amplifier is an important consideration when constructing a precision CMOS amplifier. Ideally, the operational amplifier should provide a zero output voltage absent an applied differential input voltage signal. However, due to the inherent transistor mismatches in most, if not all, operational amplifiers a zero differential input signal can produce a non-zero output signal without some form of input offset voltage cancellation to compensate for the internal offset error of the amplifier. The offset error is typically present over the entire operating range of the amplifier. Traditional offset compensation techniques may involve chopping the input signal and filtering the output voltage in continuous time applications. Unfortunately, this approach requires a high frequency signal much greater than the bandwidth of the amplifier to chop the input signal.
Another method involves storing an input offset voltage across a capacitor coupled in series with the inverting input terminal of the operational amplifier to compensate the input signal for errors induced by the amplifier mismatches. During an auto-zero phase, switching circuits are closed to configure the operational amplifier as a unity gain buffer while a reference potential is applied through the series capacitor to the inverting input terminal which appears at the output of the amplifier modified by the internal offset thereof. The input offset voltage is thereby stored across the series capacitor. During the operational phase, the reference potential is replaced with the input signal whereby the input offset voltage stored across the capacitor is subtracted from the input signal and added back as the input signal propagates through the operational amplifier thereby cancelling the internal offset error. However, any error or leakage appearing on the series capacitor affects the input offset compensation. The method of storing the input offset voltage across the series capacitor is preferable for systems using sampling since it allows time for the auto-zero phase.
The series capacitor offset compensation technique for the CMOS operational amplifier requires unity gain stability in an inverting configuration for proper operation. Non-inverting type operational amplifiers are not supported by the series capacitor cancellation. Moreover, the auto-zero phase limits the continuous time operation of the amplifier in that the input signal must be disabled during the time the input offset voltage is stored across the series capacitor. Therefore, the output signal is available only as chopped bursts.
Hence, what is needed is an improved offset voltage compensation technique for a CMOS operational amplifier in both inverting and non-inverting configurations with continuous time operation.